; Program SSC_DEMO.SRC (Mini SSC educational demo). ; Written in Parallax dialect of PIC assembly language. ; This program is a demo version of the firmware for the Mini SSC, ; a device that controls eight radio-control servos in accordance with ; instructions recieved over a 2400- or 9600-baud serial hook-up. The ; commercial Mini SSC also recognizes individual board IDs, allowing up ; to 32 controller (256 servos!) on one serial port. This demo version ; is not addressable, works at a fixed rate of 2400 baud, and controls ; just on e servo. However, it has the same basic structure as the full-up ; program. It is provided for novice PIC users interested in a method for ; Running two time-sensitive processes concurrently. ; Serial data is accepted through ra.3; servo pulses (1 to 2 ms, repeated ; approx. 60 Hz) are output at rb.0. This program runs propertly on a ; PIC16C61, 71, or 84 with an 8 MHz clock. At the time this program was ; written, the 16C61 was new, and PASMX/PEPX did not recognize it as a ; valid device. Fortunately, the '61 programs just find from code generated ; for a 71 or 84 (provided you don't use the special features of those ; devices). If you program a 61, be sure that PEPX is set for a 71 - setting ; it for an 84 will cause programming errors. device pic16c71, xt_osc, wdt_off, protect_off, pwrt_off id 'SVDM' ;Symbols (constants) used in the program. one_ms = 6 ; value to load into RTCC for 1-ms delay ser_pin = ra.3 ; Serial-input pin stb24 = 44 ; Start delay for 2400 bps @ 8 Mhz bit24 = 164 ; Full delay for 2400 bps @ 8 Mhz ; Variable declarations. Variables for this program begin at address ; 0C hex, placing them above the PIC's special-function registers. org 0Ch svo_PC ds 1 ; Program segment counter for servo ISR w_copy ds 1 ; Copy of w register made by ISRdelay_cnt ds 1 ; Counter for serial delay routines. bit_cnt ds 1 ; Number of received bits. rcv_byte ds 1 ; The received byte temp ds 1 ; Reusable temporary storeage position ds 1 ; Position value. end_frame ds 1 ; Counter for end of frame delay ; Upon reset the PIC first executes the code located at address 0. In this ; demo, it jumps to the beginning of the foreground program located at ; "start" When an interrupt occurs, the PIC jumps to address 4. This is ; where the interrupt handler is located. org 0 jmp start ; The reset vector. org 4 ; The interrupt handler. ; The interrupt-service routine (ISR). When RTCC overflows, the resulting ; interrupt sends the program here. This ISR stores the w and status ; registers, then jumps to the program segment stored in svo_PC. ; The segments load RTCC and start and stop the servo pulses ; based on the data found in the variable position. Once finished, they ; store the address of the next segment to be executed, restore the ; registers, and execute the reti (return from interrupt) instruction. do_servos clrb rtif ; Reset timer-interrupt flag. mov w_copy,w ; Save the status and W registers. mov w,<>status ; Swap status into w. mov st_copy,w mov w,svo_PC ; Move servo program couter to w. jmp w ; Jump to segment pointed to by svo_PC ; Segment zero starts the servo pulse by weiring a high to rb.0 and ; setting RTCC to interrupt again after a 1-ms delay, it loads segment 1 ; into tho "servo program counter" that do_servos used to jump to the ; correct segment of the routine. seg_0 setb rb.0 ; Initiate pulse mov rtcc,#one_ms ; Set up 1-ms delay. mov svo_PC#seg_1 ; Execute next segment at 1-ms timeout mov w,<>st_copy ; Restore the status register and W. mov status, w swap w_copy mov w,<>w_copy ; Return from interrupt. reti seg_1 mov w,/position ; Load the inverse of the position value mov rtcc,w ; into the rtcc to create delay mov svo_PC,#seg_2 ; Execute next segment after timeout mov w,<>st_copy ; Restore the status register and W. mov status,w swap w_copy mov w,<>w_copy reti ; Return from interrupt seg_2 clrb rb.0 ; End the pulse. mov w.position ; Load the non-inverted position value mov rtcc.w ; into rbcc to create a pad delay mov svo_PC,#seg_3 ; Execute next segment after time out. mov end_frame,#14 ; Wait 14 ms before seg_0 (see seg_3) mov w,<>st_copy ; Restore the status register and W. mov status,w swap w_copy mov w,<>w_copy reti ; Return from interrupt seg_3 mov rtcc,#one_ms ; Set up a 1-ms delay djnz end_frame,:cont ; Repeat this segment untill end_frame=0 mov svo_PC,#seg_0 ; When end_frame runs out, point to seg_0 :cont mov w,<>st_copy ; Restore status register and W. mov status,w swap w_copy mov w,<>w_copy reti ; Return from interrupt ; Start marks the beginning of the initialization code that sets up the ; program. It configures the RTCC to increment off the system clock ; through a divide-by-8 prescaler. At 8 MHz, that makes one RTCC tick ; equal to 4 microseconds - the desired resolution of our servo-pulse ; generator. Start also sets the servo routine's program counter to ; segment 0. start mov svo_PC,#seg_0 ; Point to base segment of ISR setb rp0 ; Switch to register page 1. clr wdt ; Transfer the prescaler to RTCC. mov option,#10000010b ; and set it for /8 clrb rp0 ; Back to page 0. mov intcon,#10100000b ; Turn on RTCC interrupt. main mov lra,#1000b ; Serial input is ra.3. mov lrb,#0 ; make rb all outputs. :start_bit ; Receive serial data. snb ser_pin ; Look for start bit. jmp :start_bit ; No start bit? keep watching. call serl_in ; Get the serial data. mov delay_cnt,#bit24 ; Stop-bit delay. call delay ; Wait for stop bit. mov posiiton,rcv_byte ; Move data to position goto main ; Endless loop. ; Subroutine to read in serial data from the port bit ser_pin. ; Note that there's no stop-bit delay. Callers may incorporate ; this delay into other operation. serl_in mov bit_cnt,#8 ; Eight data bits. clr rcv_byte ; Get ready for new data. mov delay_cnt,#stb24 ; Start-bit delay call delay ; Wait untill first data bit. :rcv mov delay_cnt,#bit24 ; Full-bit delay. call delay ; Wait for bit. movb c,ser_pin ; Put data bit into carry. rr rcv_byte ; Rotate bit into byte. djnz bit_cnt,:rcv ; Not full? Get next bit. ret ; Simple do-nothing loop for variable length dealys. Program must ; logad delay_cnt before calling this routine. delay jmp $+1 djnz delay_cnt,delay ret
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